1. Field of the Invention
The present invention relates to a stencil printing machine which transfers ink, transuding out of pores in a stencil paper, to a print medium by transferring the print medium while pressing the print medium against a printing drum on which the stencil paper is mounted.
2. Description of the Related Art
As conventional stencil printing machines of this type, there have been an inner press type (disclosed in Japanese Patent Application Laid-open No. 7-132675) and an outer press type (disclosed in Japanese Patent Application Laid-open No. 2001-246828).
The inner press type will be explained briefly as follows. The inner press type is provided with a printing drum and a back press roller whose diameters are nearly equal. The printing drum and the back press roller are arranged so as to be each capable of being rotated in a state that parts of each of the outer peripheral surfaces of the printing drum and the back press roller are made closely adjacent to each other. The printing drum, whose outer peripheral wall is flexible, is formed of an ink permeable screen. The printing drum is provided, inside the outer peripheral surface, with an ink supplying mechanism which supplies ink to the inner surface of the screen, and which can press the inner surface of the screen by an inner press roller.
Then, the printing drum on which a stencil paper is mounted and the back press roller are caused to be rotated while being synchronized with each other. A print sheet is fed between the rotated printing drum and the rotated back press roller. Thence, the inner press roller presses the screen. By the pressing, the print sheet is transferred between the back press roller and the stencil paper while being pressed against, and brought into contact with, the back press roller and the stencil paper. In the process of this transfer, ink on the side of the screen is transferred, out of pores in the stencil paper, onto the print sheet. Accordingly, an image is printed onto the print sheet.
In addition, the outer press type will be described briefly as follows. The outer press type is provided with a printing drum. The outer peripheral wall of the printing drum is formed of a porous, ink permeable member. The printing drum is provided, inside, with an ink supplying mechanism which supplies ink to the ink permeable member, and outside, with a pressure roller.
Then, the printing drum on which a stencil paper is mounted is caused to be rotated, and a print sheet is fed between the rotated printing drum and the rotated presser roller. Thence, the pressure roller presses the printing drum. By the pressing, the print sheet is transferred between the pressure roller and the stencil paper while being pressed against, and brought into contact with, the pressure roller and the stencil paper. In the process of this transfer, ink on the side of the printing drum is transferred, out of pores in the stencil paper, onto the print sheet. Accordingly, an image is printed onto the print sheet.
With regard to each of the above-described, conventional, inner and outer press types of stencil printing machines, however, an ink pool is formed in the ink supplying mechanism located inside the printing drum, and ink in the ink pool is supplied to the printing drum in the course of a printing operation. Consequently, when the printing is not performed for a long time, ink held in the ink pool and ink staining in the printing drum are left in a state of being exposed to the atmosphere for a long time. This causes a problem of the ink being changed in quality.
In addition, since various rollers for supplying ink have to be arranged inside the printing drum, this causes a problem of making it difficult to miniaturize, and to reduce the weight of, the printing drum.
Against this background, the applicant of the present invention has developed a stencil printing machine comprising: a printing drum, which is capable of being rotated, which includes an outer peripheral wall formed of ink impermeable material, and on which a stencil paper is mounted around the surface of the outer peripheral wall; an ink supplying mechanism, which includes an ink supplying unit in the outer peripheral wall of the printing drum, and which supplies ink from the ink supplying unit to the surface of the outer peripheral wall; a pressure roller which presses the fed print medium against the surface of the outer peripheral wall.
In this stencil printing machine, when the outer peripheral wall is caused to be rotated and a print medium is fed in a state that ink is supplied from the ink supplying unit to the surface of the outer peripheral wall, this print medium is transferred while being pressed against the stencil paper and the outer peripheral wall of the printing drum by the pressure roller. Concurrently, ink supplied between the outer peripheral wall of the printing drum and the stencil paper is caused, by the pressure of the pressure roller, to be spread downstream in the printing direction while being squeezed through in-between. In addition, the spread ink transudes out of pores in the stencil paper, and is transferred onto the print medium, whereby an image is printed onto the print medium. Ink which has been supplied to the outer peripheral wall of the printing drum is held in a virtually airtight space between the outer peripheral wall of the printing drum and the stencil paper, and the exposure of the ink to the atmosphere is minimized. Furthermore, various rollers for supplying ink need not be arranged in the interior of the printing drum. As a consequence, ink will not be changed in quality even when the printing is not performed for a longtime. Thus, the printing drum can be miniaturized, and the weight can be reduced.
With regard to the stencil printing machine, however, ink has not been supplied to the inner surface of a new stencil paper which has just been mounted on the printing drum. For this reason, it is highly likely that, if the same amount of ink as is used for usual printing operations is supplied to the stencil paper at this moment, the ink does not cover the entire printing surface of the stencil paper fully while an initial printing operation is performed. Since, therefore, printed sheets which include places that are not fully printed are produced, trial printings have to be repeated until printed sheets which meet a desired quality are brought about. In some cases, print sheets have been wasted.
In addition, ink which is held in the inner surface of the stencil paper is not completely free from the exposure to the atmosphere, although the exposure of the ink to the atmosphere is suppressed as little as possible as prescribed above. If, therefore, the printing drum is left in a state that a stencil paper is mounted on the printing drum for a long time, ink on the surface of the printing drum transudes out through pores in the stencil paper or does other things. In a case that printing operations are resumed while in a state that the printing drum has been left as it is for a long time, it is highly likely that ink does not fully cover the entire printing surface as long as the ink to be supplied is in the same amount as is used for usual printing operations. Since, therefore, printed sheets which include places that are not fully printed are produced, trial printings have to be repeated until printed sheets which meet a desired quality are brought about. In some cases, print sheets have been wasted.